How the Universe Works - S01E08 - Moons
Category
📺
TVTranscript
00:00In the universe, everything seems to orbit something. Planets orbit stars, and moons
00:14orbit planets. Some moons are volcanic, but the volcanoes are ice. Others are awash with
00:23great oceans. There may be more habitable moons in our galaxy than there are habitable
00:31planets. Moons tell the unknown stories of our solar system and show us how it all works.
00:53In our own solar system, there are just eight planets. But orbiting six of those planets
01:07are moons. Lots and lots of moons, more than 300 of them. Each one is different. Each one
01:30a world all its own. Well, when we look out at our solar system, we see a lot of planets.
01:37But even more than planets, we see moons. And in many ways, they're more interesting
01:41than the planets that they go around. We have moons that are airless and apparently dead
01:48like ours. Then out in the outer solar system, we have moons with oceans inside them and
01:54moons with atmospheres around them. I'm for moons. You can keep the planets. The biggest
02:01eruptions, the coldest temperatures, and the largest oceans in the solar system, they're
02:17all on moons. There are moons with ice volcanoes. There are moons with lakes of methane and
02:27methane rainfall, smog, clouds. Moons that are so volcanically active that they keep
02:35remaking their surface. Moons with all kinds of plumes shooting off into space. Really,
02:43a much wider range of environments than we ever could have imagined. Often when I'm describing
02:55to the general public or even to my fellow scientists these moons of Saturn and Jupiter,
03:00I call them worlds because they really do have the complexity and mystery of a whole
03:05planet. Jupiter and Saturn have over 60 moons each. These giant gas planets and their moons
03:15are like mini solar systems. And each moon has a distinct personality. Iapetus, a two-toned
03:27moon in black and white. Titan, with a dense orange atmosphere. And icy Enceladus, blasting
03:50ice geysers 200 miles into space. Each moon is unique. But they all have one thing in
04:10common. All moons are natural satellites held in place by gravity. But moons do more than just go
04:21around planets. They help stabilize the planets in their orbits and keep the machinery of the
04:26solar system running smoothly. The diversity of moons is an interesting combination of predictable
04:37laws of science and then complete randomness of just things smashing together and the chips kind
04:42of falling where they did in a way that you could never predict. Planets and moons begin the same
04:53way. Once a star turns on, there's a lot of dust and gas left over. Slowly, the dust particles clump
05:12together, forming rocks. The rocks smash into each other and form boulders. Slowly, the objects get bigger and
05:26bigger. The process is called accretion. One can think of it as forming a snowball and rolling it down a hill. As it
05:37rolls down the hill, it collects and gathers up yet more snow, which makes it roll faster and harder. And so that
05:44process of runaway accretion actually happens in the formation of the planets and in the formation of moons as well.
05:55It sounds simple enough, but nobody knew for sure how it worked until 2003. On the International Space Station, astronaut Don
06:09Pettit was experimenting in zero gravity. He put grains of salt and sugar inside a plastic baggie. Instead of floating apart,
06:21they began to clump together. This is how both planets and moons build up. But instead of taking shape around stars, most big
06:36moons take shape around planets. If the same process makes them all, what makes all of them so different from each other? Take two of
06:51Jupiter's moons, Callisto and Ganymede. Two very different moons, each born from the same debris when Jupiter was still young. Ganymede
07:09formed close to Jupiter, where there was lots of debris. Because there was so much material, it came together quickly, in about 10,000 years. And it was hot. The heat
07:24separated the ice from the rock. You can still see it in Ganymede's distinct landscape. The primary factor that affects why moons are the way they are today is
07:37energy. How much energy was put into them as heat during accretion, and how much energy has been lost. All of those factors go into telling us why moons behave the way they do and why they look the way they do today.
07:55Callisto's surface tells a different story. It formed much farther out, where there was less debris and less heat. It took longer and cooled faster. Unlike Ganymede, Callisto's surface is uniform. Rock and ice never separated.
08:18Where a moon forms can also mean the difference between survival and destruction. Get too close, and a planet's gravity will rip a moon to shreds. Scientists believe this is what happened to many moons when Jupiter was young.
08:48And it's very likely that Jupiter had an entire conveyor belt of large moons that were wanting to form, only to be swallowed up by the planet itself. The large moons we see today are only the last ones that were able to stabilize right at the end of that process, stop their death spiral, and survive into the position we see today.
09:12But Jupiter keeps trying to eat them. The gravity of the giant planet reaches out and pulls hard on the orbiting moons. It transforms them from lifeless balls of rock into strange and dramatic worlds.
09:42Jupiter is the largest planet in our solar system. It has 63 moons. The four largest are called the Galilean moons, named after the astronomer Galileo, who discovered them in 1610.
10:11They show how gravity controls both what moons look like and how they behave. The first of the Galilean moons, Io, orbits closest to the planet, just 260,000 miles above Jupiter.
10:41That's about the same distance as our moon is from Earth. But unlike our moon, the surface of Io has no impact craters. Scientists realized that meant the surface was new. But how could that be?
11:02Every time you look at Io with a spacecraft or even with a telescope, it's a little bit different. So the geology on Io changes like the weather on other planets. It's that active.
11:13When NASA first sent probes to fly past Io, they were shocked. They saw dozens of active volcanoes.
11:21This is footage of an erupting supervolcano on Io, blasting 200 miles into space. Everyone had the same question. How could there be active volcanoes on a moon? The answer was simple. Gravity.
11:46Jupiter's gravity is so huge that it reaches out and crunches the moon. And it's not just Jupiter's gravity pulling on Io. Other nearby moons also pull on it as they pass by.
12:04So the core of the moon is being worked back and forth all the time. It's called tidal friction and generates extreme heat in Io's core.
12:15Almost like bending a wire coat hanger until it breaks and you feel the inside of the coat hanger there. It feels rather warm. That tidal friction, that internal friction, heats the interior of Io until it's become actually one of the most volcanically active worlds in the solar system.
12:31The constant pushing and pulling generates temperatures thousands of degrees high inside Io. It blasts out in gigantic eruptions of lava.
12:42Io is the prime example of tidal forces and gravitational interactions in the solar system. It is constantly being pulled by Jupiter and it's constantly getting pulled by the other moons as well. And so as a result, there's a tremendous amount of heat created.
12:59The floods of erupting lava constantly resurface Io, which is why there are no visible impact craters on this moon.
13:14Gravity also heats Io's neighbor, Europa. Europa's orbit is farther away from Jupiter, so it's much colder. Instead of lava, the surface of Europa is ice.
13:33The lowest recorded temperature in Antarctica is minus 128 degrees. Europa's surface is twice as cold.
13:44But underneath all the ice, there may be an ocean of water heated by the same tidal friction that makes Io volcanic.
13:55Europa has a subsurface ocean, almost certainly, and that subsurface ocean is in contact with the Rocky Mantle, which provides heat and also provides probably appropriate nutrients to sustain life.
14:15Someday, we'll send a probe to explore beneath the ice on Europa.
14:24And maybe we'll discover life forms living there in warm Europan oceans.
14:36Out beyond Io and Europa are nearly 60 more moons.
14:46They orbit much further away from Jupiter, where the effects of the giant planet's gravity are much weaker.
14:55Out here, it's too weak to generate tidal friction and heat the moons.
15:06So, these remote worlds are cold and barren, but not featureless.
15:16They bear the scars of countless collisions, and scientists believe it was collisions that created the most extraordinary moon system of them all.
15:35The planet with the most unusual moon system is Saturn.
15:41It's spread out over more than 200,000 miles.
15:47Technically, there are more than a billion moons.
15:51That's right, a billion moons.
15:54And all together, they make up Saturn's rings.
16:01A moon can be a hunk of rock or ice, no bigger than a pebble, as long as it orbits a planet.
16:07The rings of Saturn are made of countless pieces of rock and ice.
16:12They go from the size of a pebble up to the size of a city.
16:17We don't refer to all the ring particles that can get to be as big as 10 or 20 meters across.
16:23We don't refer to them as individual moons.
16:26But when we find a body that is maybe a kilometer or two across, then you can start talking about it as a moon or a moonlet.
16:38Saturn's rings are one of the oldest mysteries of astronomy.
16:42Where did they come from?
16:46To try and find out, NASA sent the Cassini probe on a 12-year mission to study Saturn, its rings and its moons.
16:56We took with Cassini probably the most beautiful picture that's ever been taken, and I'm not the only one who has said this.
17:05Cassini was in the shadow of Saturn, cast by the sun, and so you don't see the sun.
17:11You see the back-lit planet of Saturn and its beautiful rings.
17:15You see the refracted image of the sun poking out from the back of Saturn.
17:20And nestled in all of that splendor is this small little dot.
17:26That tiny dot is not a moon.
17:29That is the distant planet Earth, nearly a billion miles away.
17:38Most of what we know about Saturn, of its rings and moons, comes from Cassini.
17:43Before Cassini, we thought there were only eight rings.
17:47Today, we can see over 30.
17:51What we have found at Saturn has been just literally an embarrassment of riches.
17:56We're seeing something that we had seen before, but now we're seeing it with a level of detail and clarity that was just mind-blowing.
18:04Scientists used to think the rings were made of the icy leftovers after Saturn was formed, about four billion years ago.
18:12But anything that old should be covered with cosmic dust and dirty.
18:17And that's what Cassini is all about.
18:20Cassini is the most beautiful picture of the sun.
18:23It's the most beautiful picture of the sun.
18:26It's the most beautiful picture of the sun.
18:28Anything that old should be covered with cosmic dust and dirty.
18:36So, why do Saturn's rings appear bright and clean, almost new?
18:46To get the answer, Mission Control maneuvered Cassini close to the rings.
18:51The probe saw that all the ice pieces in the rings are constantly colliding and breaking up.
19:03And each collision exposes new surfaces that are clean and polished.
19:21This is what astronomers think happened.
19:25When Saturn was young, it had no rings, just lots of moons.
19:30At some point, an icy comet zoomed in from deep space and smashed into one of those moons.
19:38The comet broke up into billions of pieces.
19:41The impact also pushed the moon closer to Saturn, where the planet's enormous gravity broke it up.
19:57Now, debris from the moon and ice from the comet mixed.
20:01Gradually, Saturn's gravity pulled all those fragments into rings around it.
20:11The story of moons is the story of gravity.
20:15Gravity holds them in orbit.
20:18It heats up their insides and shapes their surfaces.
20:23In the end, it controls everything.
20:26Gravity can even create new moons by kidnapping asteroids, comets, and even whole planets.
20:42We know that gravity makes moons.
20:45But how does it make them?
20:48We know that gravity makes moons.
20:53The standard way is to assemble them from debris left over when planets are formed.
21:02But gravity makes moons a second way, too.
21:05It captures them.
21:07Imagine a wandering comet or asteroid.
21:11Somehow, it gets knocked off course.
21:14It wanders too close to a planet.
21:18Gravity acts like a science-fiction tractor beam and grabs it.
21:24Not quite enough gravity, and it escapes.
21:28It's called a comet.
21:30Imagine a tractor beam and grabs it.
21:34Not quite enough gravity, and it escapes.
21:39Too much gravity, and it collides with the planet.
21:44Just enough, and the comet or asteroid goes into orbit around the planet and becomes a new moon.
21:52Mars has two tiny moons named Phobos and Deimos.
21:58Both are captured asteroids.
22:02One is pushing outward as it circles the planet and will eventually break free and continue on its journey through space.
22:11The other is circling inwards, a little closer to Mars all the time.
22:16Eventually, it'll smash into it.
22:27This is Cruithne.
22:29It's an asteroid, really, just three miles across.
22:32But it's sometimes described as Earth's second moon.
22:38With the little object Cruithne, which was discovered back in 1986,
22:42we start to get into this realm of, what does it mean to be a moon?
22:49Only a few thousand years ago, Cruithne was an ordinary asteroid orbiting the sun like billions of others.
22:56But eventually, it wobbled out of its orbit in the asteroid belt and got snagged by Earth's gravity.
23:05But then, Cruithne did something unusual.
23:08Instead of orbiting around the Earth like a normal moon, Cruithne began to follow behind it.
23:15And so, one might call it a sort of a moon of the Earth.
23:19Not exactly, though, because that object is on its own independent orbit around the sun, not the Earth.
23:29Sometimes, asteroids capture their own moons.
23:34Sometimes, asteroids capture their own moons.
23:38In 1993, the Galileo spacecraft flew past the asteroid Ida and found something nobody expected.
23:47A tiny, half-mile wide moon.
23:52The fact that we saw a satellite around only the second asteroid ever to be encountered with a spacecraft
23:58immediately tells us that moons around asteroids must be incredibly common.
24:04Not all captured moons are small.
24:09The mother of all captured moons is Triton.
24:13It orbits the planet Neptune, and it is big.
24:18About 1,700 miles in diameter.
24:21But Triton is a moon with an unusual story.
24:27Triton was a very puzzling problem for planetary scientists
24:31because our traditional view would tend to make all the moons orbit in the same direction that the planet itself spins.
24:38In the case of Triton around Neptune, it's the other way around.
24:41Neptune is spinning this way, Triton is orbiting around in the opposite direction.
24:47This means it didn't form like most moons, out of the debris left over from the birth of the planet.
24:53Or, it would orbit in the same direction.
24:56So something wasn't right.
25:00Triton is huge, and its orbit is funny, it's anomalous.
25:04It does not seem as though it formed as a part of the Neptune system.
25:10It seems much more like a captured planet.
25:16Scientists now think Triton was once a dwarf planet, like Pluto.
25:20And a giant planet like Neptune certainly has enough gravity to capture a moon the size of Triton.
25:29Triton was almost certainly formed way out in the outer solar system, and then at some point was captured by Neptune.
25:35Perhaps Triton early on had its own moon.
25:37They both were captured, and then that moon was destroyed during the capture process.
25:45But Triton is in danger.
25:47Neptune is dragging it closer and closer.
25:53Eventually, it will get too close.
25:57And Neptune's immense gravity will tear it apart.
26:13Triton, the moon, will be reborn as a ringed system around the planet.
26:27But what about our moon?
26:29How did it get there?
26:31Was it captured?
26:37The truth is even more extraordinary.
26:40It was born in extreme violence.
26:49Our moon, like Neptune, is a ringed system.
26:54Our moon, like a lot of moons, is rocky, barren, and pockmarked with craters.
27:03But in one way, our moon is unique in the solar system.
27:12For a long time, astronomers thought the moon formed from debris left over from the birth of the Earth.
27:19But researchers in the 1960s came up with a radically different idea.
27:24They suggested it came from a giant impact.
27:42When we first had the idea of forming the moon from a giant impact,
27:48that was not a terribly popular idea.
27:52And I actually did have good science friends, colleagues, coming to me saying,
27:56you know, we really have to exhaust all the slow evolutionary theories
28:01before we start talking about cataclysms.
28:04The evidence Bill Hartman needed was on the moon itself.
28:09And the proof had to wait until Apollo astronauts finally went there in 1969.
28:18They brought back hundreds of pounds of moon rocks.
28:24Scientists analyzed the rocks and were amazed.
28:28They were identical to rocks in the Earth's crust.
28:32And they'd been superheated.
28:39So how did pieces of the Earth's crust become superhot and wind up on the moon?
28:45Hartman was pretty sure he knew.
28:48This whole idea was that the Earth forms, now you hit it with something,
28:52you blow all this light rocky material off the top,
28:55that material goes into orbit and makes the moon.
28:58The moon's just made out of rocky debris.
29:02Imagine our chaotic solar system four and a half billion years ago.
29:15The young Earth is just one of a hundred or so new planets orbiting the sun.
29:21One of them is a Mars-sized planet called Theia,
29:25and it's on a collision course with Earth.
29:33They smash into each other at many thousands of miles an hour.
29:51Theia is destroyed, and Earth barely survives.
29:55The impact blasts billions of tons of debris into space.
30:00The Earth's gravity pulls it into orbit around the planet.
30:04Now these hunks of leftover Earth clump together and form our moon.
30:11That's the theory anyway, but how do you test it for real?
30:19Here at NASA, we're actually going to test it for real.
30:23We're going to go to the moon and see if it's really there.
30:27And we're going to see if it's really there.
30:30That's what we're going to do.
30:32We're going to go to the moon and see if it's really there.
30:35That's what we're going to do.
30:38Here at NASA's Vertical Gun Range,
30:41they're recreating that ancient collision in a lab.
30:47This 30-foot long gun fires a tiny projectile at 18,000 miles an hour.
30:56The projectile is Theia.
30:58This ball represents the Earth.
31:01By changing the angle of Theia's impact,
31:04the team can figure out how precise the ancient collision had to be
31:08in order to make the moon.
31:11In the first shot, Theia hits the top of the Earth with a glancing blow.
31:16So here's the Earth, if you will.
31:18It's suspended in space, and now it's gotten hit.
31:23So now we see the planet, the ejecta,
31:26is being ripped out of the Earth and is forming this giant impact basin.
31:32And if this really were the Earth,
31:34this basin would be thousands of kilometers, thousands of miles across.
31:40In this simulation, Theia only skims off the surface of the planet,
31:45and very little debris is thrown out into space.
31:50Not nearly enough to build our moon.
31:54The second shot is a head-on collision.
32:02Kapow! That's the end of planet Earth.
32:05It's gone.
32:07Some of the debris is going to go out of the solar system.
32:10Some of the debris will reaccrete
32:12to form small planetesimals within the solar system.
32:17There's no Earth left,
32:19so there's no gravity to gather the debris and form the moon.
32:25Now, the gun is set to just the right angle,
32:29halfway between a glancing blow and a direct hit.
32:33So we'll see what happens if the Earth barely survives.
32:37We'll see what happens if the Earth barely survives.
32:42Oh! Oh, gorgeous!
32:44Oh, my gosh! Kapow!
32:47Now we have the entire part of the Earth being ripped apart.
32:51But the vapor plume is... Oh, my gosh!
32:54Ah! Jeez!
32:57That is gorgeous.
32:59Now we're going to see what happens
33:01if the Earth barely survives.
33:03Kapow!
33:05Kapow!
33:07Kapow!
33:09That is gorgeous.
33:17But this was the beginning,
33:19the beginning of our moon.
33:25The experiment shows that Theia could have smashed
33:28into the Earth and formed the moon.
33:32But the collision had to be just right,
33:35and lucky for us, it was.
33:40Today, the moon orbits 250,000 miles from Earth.
33:46But when it first formed,
33:48the moon orbited just 15,000 miles above the Earth's surface.
33:56500 million years after the moon formed,
33:59if we looked up in the sky,
34:01the moon would have comprised a tremendous portion of the sky.
34:05It would have been enormous,
34:07because the moon would have been much closer.
34:10Back then, the Earth was rotating so fast,
34:13a day lasted just six hours.
34:19But the moon was so close,
34:21its gravity acted like a brake.
34:28It slowed our planet down
34:30until a day now lasts six hours.
34:35That's 24 hours.
34:40The moon's gravity also created giant tides
34:43that surged across the planet,
34:45churning up the seas, mixing minerals and nutrients.
34:50This created the primordial soup
34:52from which the first forms of life arose.
34:55Without our moon, life on Earth may never have happened.
35:00And there may be other moons with a link to life as well.
35:05Moons may be the great biology experiments of the universe,
35:10the true laboratories of life itself.
35:21Moons are full of surprises.
35:24There are moons with giant volcanoes,
35:27moons with vast oceans sealed under thick ice.
35:35And now we know a few are rich in organic compounds.
35:39In the right combination, they might even support life.
35:44In our solar system, the biological window
35:47through which we can understand the rest of the universe
35:50may be through these moons of the outer solar system.
35:54That may be where we find our second genesis,
35:57and that second genesis is really our first deep understanding
36:01of the biological nature of the universe.
36:12At first glance, moons don't look ideal for life.
36:18Take Enceladus.
36:21It's a shiny ball of ice 300 miles across, orbiting Saturn.
36:29It's the brightest object in the solar system.
36:32It reflects 100% of the light that hits it, so it's super bright,
36:36and that's because it's water ice.
36:39In 2005, the Cassini probe spotted ice volcanoes
36:43erupting from the surface of Enceladus.
36:46That meant there had to be heat under all that ice,
36:49heat that created oceans of water.
36:52And where there's water, there's the possibility of life.
36:58So this is Beehive Geyser here in Yellowstone,
37:01and it is shooting water vapor and water
37:04about 150 feet into the sky, and it's pretty incredible.
37:08So now imagine if you're on the surface of Enceladus.
37:11You would see geysers that look a lot like this.
37:14And they are shooting ice grains and water vapor
37:17into space thousands of times higher than this geyser here.
37:22The ice volcanoes are powered by gravity.
37:26Here's how.
37:28Saturn's gravity works on the core of the moon, heating it up.
37:32The underground water expands and forces its way
37:36up through cracks in the surface ice
37:39and blasts out into space as ice crystals.
37:43These are some of the most spectacular eruptions
37:47in our solar system.
37:49They make Beehive Geyser look like a squirt gun.
37:53From the ice in the volcanoes,
37:55scientists have detected salt and simple organic compounds.
38:00That means the water under the ice
38:03is not only warm but full of nutrients.
38:07Sound familiar?
38:09Heat, water, and nutrients.
38:12That's how life on Earth began.
38:14We realized you could have all the things
38:16that we associate with oceans on the Earth
38:19going on inside a moon.
38:21It's the discovery of a lifetime.
38:24Saturn's Enceladus has an ocean.
38:27So does Jupiter's Europa.
38:29But these aren't the only moons where life could emerge.
38:34Saturn has another moon, Titan,
38:37with an even greater potential for life.
38:43In 2005, Cassini sent a probe called Huygens
38:47on a one-way mission to Titan.
38:52For just 3 1⁄2 hours,
38:54Huygens transmitted live pictures from the hostile surface
38:58nearly a billion miles away.
39:02Then the battery died.
39:06It was just incredible.
39:08This was the first time humans had ever touched this moon
39:11with something of our own making.
39:13It was just an event that should have been celebrated the world over.
39:17We should have had ticker tape parades in every major city
39:20across the U.S. and Europe to celebrate this.
39:23It was that history-making and that astonishing.
39:34Raindrops on Titan are twice as big as raindrops on Earth.
39:40But the rain isn't water.
39:43It's methane.
39:50On Earth, methane is a gas.
39:53But on Titan, it's a liquid because the moon is so cold.
40:01There may be methane icebergs.
40:03There's certainly methane lakes and rivers,
40:05and there's methane rain and methane clouds,
40:08and maybe bugs swimming in methane.
40:12Bugs living in liquid methane may sound unbelievable.
40:17But scientists have discovered that Enceladus, Europa, and Titan
40:22are all covered with a substance called tholin.
40:26Tholin contains the chemical building blocks for life to begin.
40:31So, could life emerge on any or all of these moons?
40:39We can't get our hands on the tholin from the moons,
40:42so Chris McKay makes it in the lab.
40:45He zaps a mixture of gases found on Titan with electricity.
40:51What he gets is a reddish-brown mud.
40:55So this is what we make, tholin,
40:57this sort of non-biological organic material.
41:01It's produced by chemical energy put into simple molecules
41:04like methane and nitrogen, and here we got it.
41:07And that's the material we see on Titan.
41:10We see evidence for something like this on Enceladus.
41:14We see it on the surface of many of the moons in the outer solar system.
41:18This is nature's recipe for making the stuff that life eventually emerges from.
41:26Somewhere in the outer reaches of our solar system,
41:29on some remote moon, life may have already emerged.
41:35But it probably won't be life as we know it.
41:39Life 2.0 doesn't necessarily have to have the same genetics as life 1.0.
41:44In fact, the more different it is, the more interesting it is.
41:49Whether it's the same or very different,
41:52the discovery of life on the moons of our solar system
41:56will change the way we look at the universe.
42:01I think that should we ever find that life had originated
42:06not once but twice in our solar system,
42:10then you can easily dismiss any arguments
42:14that say that life is unique to the Earth.
42:20Moons are small, but they're still diverse and dynamic worlds.
42:27They help us understand how the universe works.
42:31They're essential cogs in the cosmic machine.
42:35Without any moons, our solar system would be a very different place.
42:40Without our moon, life may never have evolved on Earth.
42:45And who knows?
42:47When and if we find new life somewhere else in the universe,
42:51its home may not be another planet at all.
42:55It might be a moon.
42:58NASA Jet Propulsion Laboratory, California Institute of Technology